Hematopoietic stem cell (HCT) transplantation offers curative therapy for a variety of malignant and nonmalignant disorders. It is limited by donor availability, transplant related toxicity, graft vs. host disease (GVHD), malignant relapse, infections, and for some patients, reduction in their post-transplant quality of life. Continuing progress from the previous funding period, the Blood and Marrow Transplant Clinical Trials Network (BMT CTN) proposes to develop and execute scientifically meritorious, prospective clinical trials addressing key issues in HCT. We will conduct multicenter Phase II and more importantly, prospective Phase III trials in six key areas including: alternative donors and graft sources;regimen related toxicity;GVHD;disease recurrence;infection and immune reconstitution;and late effects and quality of life. Due to the complexity of pediatric transplantation, especially in those with rare inherited disorders, which are complex and difficult to study, we will devote special attention and focused scientific expertise to using the coordinated strength of the Network to improve BMT outcomes for this unique population. The BMT CTN will also amplify and leverage Network resources through collaborative research and ancillary studies of biologic endpoints that will be integrated with and complement clinical endpoints. The Network will also seek active collaboration with other scientific bodies including NCI- funded Cancer Cooperative Groups to improve the efficiency of clinical transplant studies for the large number of patients who could benefit from BMT and to maximize our successful completion of high quality and high priority clinical trials. (End of abstract.)